An increasing demand exists for pre-fabricated structural panel materials, such as plywood, wafer board, oriented strand board, plaster board, composites of veneer and of wood-based layers, and so forth. These structural materials are heavily used, for instance, in the construction and manufacturing industries. Suppliers and users of these products often need to know their design values or capacities to assist them in making a proper material selection. For instance, among other tests, specimen or control panels cut from large panels or pulled from a given production lot or product type is tested to determine the major mechanical properties of the panel material.
In the case of many viscoelastic materials, such as wood composite products, the loading rate, in pounds/minute or inches/minute of tests is carefully prescribed to allow the comparison of test results from different machines, dates, and test labs. In general, as the loading rate increases, there will be an increase in the apparent strength of the material being tested. This is mainly from the time-dependant response of the viscous portion of the material. A fully elastic material should not exhibit any changes from loading rate to the same magnitude as that of a viscoelastic material.
In the testing of wood composite panel products, the long-term performance of structural panels is important to allow the designers to get a feel for the time-dependant increase in deflection, or creep, that most products exhibit. “Creep” is the name given to the time-dependant increase in deflection that most viscoelastic materials exhibit under sustained loading. In the same time, its strength must be adjusted for the intended load duration. Creep behavior in wood products can often be seen in old bookshelves: after many years with the constant weight of books on the shelves, they tend to sag. In the ASTM Standard for creep testing of wood products, i.e., ASTM D6815-02a, Standard Specification for Evaluation of Duration of Load and Creep Effects of Wood and Wood-Based Products, ASTM Int'l, West Conshohocken, Pa., U.S.A., a 12 inch×40 inch piece of wood panel is loaded in “third-point” bending for 90 days in a controlled constant temperature and humidity environment. During the 90 day load period, a constant load is applied, normally by hanging a known weight on a cable that is attached to the loading head, which transfers the load to the wood panel specimen. The test starts with matched sets of panels, some of which are loaded in static bending, to failure at around 5 minute duration. The load rate is approximately 0.035 inch/min. The 5th percentile of the failure stress is calculated from the short-term (5 minute) testing results. The 90-day testing specimens are loaded to the same stress level as prescribed in the testing standard, and they have to be loaded at the same loading rate as the short-term bending tests. This restriction is in place so that the test operators will not knowingly or unknowingly change the loading rate from the rate of the short-term test.
Under certain protocols of ASTM D6815-02a, the load has to be applied very precisely in order to make accurate engineering extrapolations regarding board performance. Wood composite boards typically are viscoelastic materials. As a consequence, the strength of the material changes if the loading rates are different. This load application has most commonly been done by applying the load very slowly, which allows more precision and control over how the load is applied.
In order to strive for such a slow initial loading rate, many varied approaches have been explored. Many years ago, technicians applied the load using a piece of lumber held at each end by two people. These two people would try to slowly lower the weight manually until it hangs from the cable to transfer the load to the specimen. This was not very exact, so other approaches were developed. One approach was to divide the amount of total weight being applied into many smaller separate weights which were loaded on the cable one at a time until the desired aggregate load is provided. These weights could be placed in an intermittent timed manner into a bucket attached to the cable, thereby arriving at the full amount of load in the time prescribed by the ASTM standard. The problem with this additive approach was that the load was not transferred very smoothly to the cable, or in other words, it was more of a step function loading. Another technique has involved placing a screw jack under the load or airbags, and slowly lowering the weight. In some locations, a motor-driven pulley was developed to lower the weight at a certain rate. The system using a motor-drive pulley had the disadvantage of the need to hang another cable to the weight stack, and having to unhook it from each weight stack before moving to the next specimen that needs to be loaded. This added step might introduce some error to the data collection as well as take longer to apply load to many test panel samples. An added disadvantage of the overhead pulley system is that it is larger and more cumbersome to maneuver in tight areas between rows of creep test frames. As a result, engineering extrapolations often are not reliable, and the panel must be “over-engineered” to insure an adequate margin of performance capability.
Prior panel performance testing systems are known which combine panel support, load application, and system control means as an integrated single system positioned at a fixed location for testing panels one-at-a-time. Some panel bending tests, such as creep testing, may require load/deflection testing and data acquisition that lasts extended periods of time. For instance, a constant stress period of 90 days (or even longer for non-decreasing creep rate instances) is stipulated in ASTM D6815-02a.
U.S. Pat. No. 6,053,052 describes a performance testing system for wood-based panels. The testing includes performance of a material under a load concentrated in a single area, performance of edge support systems under a concentrated load and performance of a material under static bending conditions. The system is computerized and automatically applies a load to a panel to be tested using a hydraulically actuated system supported on the panel support frame. The system reads and records deflection of the panel, and provides a printed test report. U.S. Pat. No. 5,187,987 describes a bending beam creep test device in which the test specimen and lower part of a loading mechanism are submerged in a constant temperature liquid coolant. The specimen is supported on two spaced apart support members and a loading head actuated by an air bearing/pneumatic piston mechanism engages the specimen midway between the support members. The deflection of the specimen is measured with a linear variable differential transformer and the load imparted to the specimen by a load cell provided under at least one of the specimen support members.
A panel performance testing device which can be operated on-site that provides accurate and repeatable creep and DOL performance and facilitates concurrent testing of a plurality of test panels would be highly desirable and useful.